CONGRESS IS BACK IN SESSION NOW, AND Newt Gingrich--remember him?--is chewing over numbers. For Gingrich the House Speaker, there are all the usual budget totals and legislative head counts to think about. But for Gingrich the Party Strategist, there's just one big number: 35%. That's the high-end estimate of how many Republicans go for Pat Buchanan. What worries the G.O.P. leadership is that some of them might leave with him too should the party turn its back on them. Even if Buchanan doesn't become the Republican nominee--and G.O.P. leaders are still convinced It Can't Possibly Happen--the forces that Pat has mobilized...